In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. The films are made from a variety of substances, and can be conductive or non-conductive. Conductive films are typically used for wiring or wiring connections. Non-conductive or dielectric films are used in several areas, for example, as interlevel dielectrics between layers of metallization, or as isolations between adjacent circuit elements.
Typical processing steps involve: (1) depositing a film, (2) patterning areas of the film using lithography and etching, (3) depositing a film that fills the etched areas, and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP). Films are formed on a substrate by a variety of well-known methods, for example, physical vapor deposition (PVD) by sputtering or evaporation, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD). Films are removed by any of several well-known methods, including, chemical-mechanical polishing, dry etching such as reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching.
It is extremely important with removal of films to stop the process when the correct thickness has been removed. In other words, during the removal of films, it is important to know when the endpoint has been reached. In CMP, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a chemically reactive slurry. Overpolishing of a film results in yield loss, and underpolishing requires costly rework. Accordingly, various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped.
The prior art methods for CMP endpoint detection suitable for all types of films involve the following types of measurement: (1) simple timing, (2) friction or motor current, (3) capacitive, (4) optical, (5) acoustical, (6) conductive and (7) chemical. In particular, chemical endpoint detection (e.g., Li et al., U.S. Pat. No. 6,021,679) has been popular due to its ability to provide real-time and continuous analysis of the slurry during polishing, a direct signal endpoint as soon as the nitride layer is polished and a fast response time, typically less than one second, in addition to other benefits.
It has been discovered that when chemically-mechanically polishing a substrate with a target film of oxide (SiO2) over a stopping film of nitride (Si3N4) with a slurry containing potassium hydroxide (KOH), a chemical reaction occurs when the oxide/nitride interface is reached, resulting in the production of ammonia (NH3). When polishing oxide, the following reaction occurs:SiO2+2KOH+H2O→K2SiO3+2H2O
When polishing nitride, the following reaction occurs:Si3N4+6KOH+3H2O→3K2SiO3+4NH3 
The ammonia produced is dissolved in the slurry and it exists primarily in the form of NH3 rather than NH4+. Thus, the presence of ammonia in the slurry indicates that the underlying nitride film has been reached and polished, and the endpoint for removal of the oxide film can be determined by monitoring the level of ammonia in the slurry. Once the endpoint is reached, the polishing is stopped.
Typically, in order to detect and monitor ammonia in a gaseous form, slurry from a polishing apparatus is pumped through an ammonia extraction unit. The ammonia-containing gas stream can be analyzed and monitored for endpoint detection for removal of the target film. Gas phase chemical analysis, such as standard mass spectroscopy can be highly sensitive and have a fast response time, that would be desirable for endpoint detection. Unfortunately, with slurry sampling, there may be substantial interference from any residual ammonia created from the slurry composition itself, making accurate endpoint detection extremely difficult.
Hence, what is needed is a composition and method for chemical-mechanical polishing of silica and silicon nitride for shallow trench isolation processes having improved end-point detection capability.